1.about which这是一个限制性定语从句,后面的about which i know nothing 是修饰geometry.正常的语序是i know nothing about which,其中 which在这里指代geometry,且在从句中做about的宾语.
2.如果用which在句子中要充当成分,而i know nothing,主、谓、宾都有了,不缺少任何成分,所以不能用.